The present invention relates to a method and an apparatus for optically recording information and to a recording medium therefore, and more particularly to those which permit overwriting using a single beam and which can provide a high overwrite erasing capability.
As a technique of optically recording and reproducing information, it is known to use a phase-change film as a recording medium, and change laser irradiation power to cause the phase change thereof, thereby changing the optical property thereof so as to record and reproduce information. Among techniques of using the phase-change film, the technique of overwriting new data on old data while modulating the irradiation intensity of a single laser beam is disclosed in technical search reports in the Japanese literature, "DENSI JOHO TSUSIN GAKKAI, SINGAKU GIHO", Vol. 87, No. 310 CPM87-88, 88, 89, 90.
The operation of the prior art single beam overwrite technique disclosed in the references is summarized as follows. A laser beam having the power profile as shown in FIG. 22 is irradiated onto the optical disk (recording medium) having a phase-change recording film, as shown in FIG. 21. The power profile of this laser beam is such that the pulsating power P.sub.1 for recording information at a recording position which is to be amorphous is superposed on the D.C. power P.sub.2 for recording recording information at a recording position which is to be crystalline.
FIG. 23 shows a relation between the pulse width and power of laser radiation for the recording film and the phase change thereof. As seen from the figure, with a predetermined pulse width, by varying the irradiation power, the recording film can be changed to be crystalline or amorphous. Hereinafter, making or becoming crystalline is referred to as "crystallizing" and making or becoming amorphous is referred to as "amorphousizing".
The prior art technique discloses the basic operation of implementing the phase-change single beam overwrite but has the following problem. No consideration is given to the property of the recording film and the recording condition which are required to provide a high erasing ratio, i.e. for the providing of sufficiently high erasing ratio required to actually perform a phase-change single beam overwrite. Thus, the prior art provides a low erasing ratio, and so a large error rate due to incomplete erasure of old data. Accordingly, the prior art cannot satisfy the required reliability.
The above problem will be explained in detail below. The recording process in the phase-change recording system is roughly represented by the crystallization state of the recording film. The crystallization state can be expressed by formula of JMA (Johnson, Mehl and Avrami) as disclosed in the reference of Japan Journal of Applied Physics, Vol. 26 (1987) Supplement 26-4.
The crystallization time .tau.(T) at the temperature T(K) of the recording film can be expressed by EQU .pi.(T)=.nu..times.exp(Ea/kT+Q/((Tm-T).times.(Tm-T)xT) (1)
where
.nu.: crystal nucleus frequency factor Ea: activation energy PA1 k: Boltzman constant PA1 Tm: melting point of the recording film PA1 Q: reaction constant number.
FIG. 24 shows the relation between the temperature T and the crystallization time .tau.(T) in Equation (1). In this figure, the temperature (abscissa) is represented by 1000/T. In a low temperature region on the right side of the figure, according to the increase of temperature, the crystallization time .tau.(T) is shortened. However, in contrast in a temperature region in the neighborhood of the melting point Tm, according to the increase of temperature, the crystallization time .tau.(T) is lengthened since the dissociation probability of atoms is increased as the temperature approaches the melting point Tm of the recording film.
Namely, Equation (1) shows that the temperature where the crystallization time .tau.(T) is shortest and so the recording film is easily crystallized is located at a temperature slightly lower than the melting point Tm. This temperature where the crystallization time .tau.(T) is shortest is referred to as a nose temperature Tn since in the graph of FIG. 24 it looks like a nose tip.
At the nose temperature Tn, EQU .delta.(.tau.(T))/.delta.T=0
Tn is the root of the following Equation (2) when the differentiation of Equation (1) is set at zero. ##EQU1## The root is given, as shown in FIG. 25, by a T coordinate at the intersecting point of the third order curve having a coefficient of -1 passing a coordinate (Y, T)=(0, Tm) Y=(Tm-T) and a line having a gradient of 3.times.Q .times.k/Ea passing a coordinate (Y, T)=(0, Tm/3) EQU Y=3.times.Q.times.k/Ea.times.(T-Tm/3).
As the reaction constant number Q becomes large, the gradient of the line becomes large, so that the intersecting point is shifted towards T : small. On the other hand, as the activation energy Ea becomes large, the gradient of the line becomes small, the intersecting point is shifted T : large.
FIG. 26 shows the relation between Q.times.k/Ea and the nose temperature Tn obtained using the formula of Cardann.
The crystallization rate X when the temperature is held at the temperature T for a time t is represented by EQU .delta.X=1-exp {-(.delta.t/.tau.(T)).sup.n } (3)
where n: reaction constant. In the phase-change recording film, n=2.about.3.
The crystallization of a recording film when it is heated from room temperature to high temperatures through laser irradiation thereto accords with the integration of the crystallization rates at the respective temperatures in Equation (3) in accordance with the temperature profile.
In a phase-change optical disk, laser heating is completed in a short time, within 1 microsecond, and the maximum heating temperature exceeds the melting point, so that the crystallization through the temperature profile occurs in the neighborhood of the nose temperature Tn where the crystallization time .tau.(Tn) is shortest. In the temperature range where the crystallization time is longer by one order of magnitude or more than the crystallization time .tau.(Tn) at the nose temperature Tn, the approximation .tau.t/.delta.(T)&lt;1 can be taken, and so the crystallization rate .delta.X is substantially zero. Therefore, this temperature region does not contribute to the crystallization. The temperature region participating in the crystallization where the crystallization time .tau.(T)&lt;(10.times..tau.(Tn)) is referred to as a crystallization temperature region or zone.
FIGS. 27 to 29 show changes of the nose temperature Tn and the crystallization temperature region relative to changes of the activation energies Ea (0.5 eV, 1 eV, and 2 eV) and the activation constant Q at Tm=600 C, respectively. As seen from the figures, with the increase of Q, Tn is lowered, while with the increase of Ea, the width of the crystallization temperature region is narrowed.
FIG. 22 shows the power profile of the irradiated laser during the single beam overwrite. This profile consists of two levels of power, a power level P.sub.2 continuously light-emitting for crystallizing and a power level P.sub.1 for amorphousizing superposed with recording pulses thereon.
FIG. 30A and 30B schematically show the temperature of the recording film when the crystallizing power P.sub.2 and amorphousizing power P.sub.1 are irradiated in accordance with the laser power profile shown in FIG. 22, and the crystallization rate represented by the previously mentioned formula of JMA. The rotation number of the disk is set at 1800 rpm, the linear speed is set at 10 m/s, and the laser spot diameter is set at 1 .mu.m.
Now, consideration will be given to the passing time required for the recording film to pass the crystallization temperature region in the neighborhood of the nose temperature Tn. In the crystallizing mode where the laser emits light continuously, the time t.sub.c 1 passing the crystallization temperature region, which is the time when the laser spot passes at the linear speed of 10 m/s, is about 0.1 .mu.s.
On the other hand, in the amorphousizing mode where pulsating power is superposed on the irradiation power P.sub.2 of the laser beam, the resulting temperature profile is the temperature profile in the crystallizing mode superposed with the temperature profile corresponding to the pulse component. Since the recording film is melted at the temperature of the melting point or greater, the time required to pass the crystallization temperature region has only to be considered on the cooling process after the melting. Thus, the time t.sub.c 2 passing the crystallization temperature region is about 0.05 .mu.s, which is about 1/2 of t.sub.c 1.
However, unlike the double beam overwrite, the single beam overwrite can not change the laser spot diameter between the amorphousizing mode in which the recording position is made amorphous and the crystallizing mode in which the recording position is made crystalline, the passing time in the crystallization temperature region varies by only about a factor of two between the amorphousizing and crystallizing; thus, it can not greatly vary.
Consideration will be given here about the crystallization rate in the amorphousizing and crystallizing modes. Although the crystallization is strictly represented by the integration of the crystallization rates in accordance with the temperature profile, it can be roughly determined by the t.sub.c /.tau.(Tn) (t.sub.c : passing time required for the temperature of the recording film to pass the crystallization temperature region in the neighborhood of the nose temperature Tn: crystallization time .tau.(Tn) at the nose temperature). With the passing time t.sub.c &gt;.tau.(Tn), the crystallization rate is 100% (complete crystalline phase). And, with the passing time t.sub.c &lt;.tau.(Tn), the crystallization rate is 0% (complete amorphous phase). The crystallization time .tau.(Tn) is previously defined with the recording film, and the passing time t.sub.c varies only by about two as mentioned above. Therefore, the crystallization in the crystallizing and amorphousizing modes in accordance with the power profile of the single beam overwrite as shown in FIG. 22 can not provide the crystallization rate of 100% in the crystallizing mode and that of 0% (amorphous) in the amorphousizing mode, as shown in FIGS. 30A and 30B.
If the recording film having short crystallization time .tau.(T) is selected in order to provide the crystallization rate of 100 %, the passing time t.sub.c in the crystallizing mode i.e. t&gt;.tau.(Tn) is satisfied. However, since the passing time varies by only two between the amorphousizing and crystallizing modes, the passing time t.sub.c &gt;:(Tn) results also in the amorphousizing mode. Thus, although the amorphous phase is intended to be provided in the recording mode, the crystallization rate at the recording position becomes almost equal to that in the crystallizing mode and thus, there is no difference in the crystallization rate between the crystallizing and amorphousizing modes. Since recording/reproduction is based on the difference in crystallization rate leading to a difference of a reflection coefficient, the absence of a difference in the crystallization rate substantially makes it impossible to record and reproduce signals.
In order to carry out the recording so as to provide the necessary reproduction signal level, it is necessary to set the crystallization time .tau.(Tn) of the recording film at a moderately high value ta, which is an intermediate value between the passing time t.sub.c 1 in the crystallizing mode and the passing time t.sub.c 2 in the amorphousizing mode (.tau.(Tn)=ta). Therefore, it is impossible to attain a crystallization rate of 100 % in the crystallizing mode. In FIGS. 30A and 30B .tau.(Tn) at the nose temperature Tn is set at 40 nanoseconds, which results in
the crystallization rate in the crystallizing mode: 80% PA0 the crystallization rate in the amorphousizing mode: 40%
The conventional single beam overwrite is involved in the above restrictions. Now, explanation will be given for the crystallization rate in the amorphousizing mode and that in the crystallizing mode in the overwrite under the above restrictions.
In the amorphousizing mode, the recording film at the recording position is melted to enter the liquid phase and is recrystallized in an abrupt cooling process from the liquid phase to the solid phase so that its crystallization rate remains constant and is 40%. On the other hand, in the crystallizing mode, the film is not melted, and one round process of the crystallizing mode can not provide a complete crystalline state with the crystallization rate of 100 % so that the influence of the old crystallization rate of the recording position before new crystallization is left as a previous history. If the previous phase at the recording position is crystalline as previous history, the crystallization rate has already reached 80%, and the remaining 20% part of amorphous portion which is not still crystallized is facilitated to be crystallized. The crystallization rate of the amorphous portion (uncrystallized portion) is 80% when the new crystallizing is carried out by once irradiating laser. Thus, the new crystallization rate after having experienced the crystallizing mode becomes 96% (the initial crystallization rate of 80% plus the crystallization facilitating rate of 16% for the uncrystallized portion). If the previous history has experienced the amorphousizing mode just before, the initial crystallization rate is 40%, and the new crystallization rate after having experienced the crystallizing mode this time becomes 88% (the initial crystallization rate of 40% plus the crystallization facilitating rate of 48%).
In this way, in the crystallizing mode, even though the recording film is in a crystalline phase, the film has portions with different crystallization rates of 96% and 88% which depend upon the previous history. This difference leads to incomplete erasing, which gives rise to a problem of providing an insufficient erasing ratio of the old data history in the overwrite.
This problem is particularly noticeable in an optical disk in a CAV (Constant Angular Velocity) system with constant rotating speed in which the linear speed is different in the inner and outer peripheries. The laser irradiation time for crystallizing the recording position is limited within the time t of a laser spot passing through the recording position. The time t is defined by t=(laser spot diameter)/(linear speed), and the laser spot diameter is set constant. In the CAV disk, the linear speed in the outer periphery is relatively large so that the laser irradiation time for crystallizing the recording position is shortened. Thus, the crystallization in the crystallizing mode can not be sufficiently facilitated so that the erasing ratio is disadvantageously lowered, particularly in the outer periphery. Further, in the prior art, the irradiation power P.sub.1 in the amorphousizing mode and the irradiation power P.sub.2 in the crystallizing mode are set at 20 mW and 10 mW, respectively (P.sub.1 /P.sub.2 =2) so that the width of the heating region at recording positions is varied in a track width direction for the amorphousizing mode and crystallizing mode. This also leads to incomplete erasure.